The present invention relates to a polishing apparatus for polishing a workpiece, such as a semiconductor wafer, so as to enable the workpiece to have a flat and mirror-finished surface.
With recent rapid progress in technology for fabricating high-integration semiconductor devices, circuit wiring patterns have been becoming increasingly fine and, as a result, spaces between wiring patterns have also been decreasing. As wiring spacing decreases to less than 0.5 microns, the depth of focus in circuit pattern formation in photolithography or the like becomes shallower. Accordingly, surfaces of semiconductor wafers on which circuit pattern images are to be formed by a stepper are required to be polished by a polishing apparatus to an exceptionally high degree of surface flatness. To accomplish such a high degree of surface flatness, it has become common to use a polishing apparatus known as xe2x80x9cCMPxe2x80x9d or xe2x80x9cChemical Mechanical Polisherxe2x80x9d.
FIG. 9 shows a main part of an example of a conventional polishing apparatus for CMP. This apparatus comprises a rotatable polishing table (turntable) 122 having a polishing cloth 120 adhered to an upper surface thereof. The polishing apparatus also comprises a wafer holder 124 for holding a substrate W to be polished, such as a semiconductor wafer, and an abrasive liquid supply nozzle 126 for supplying an abrasive liquid Q to the polishing cloth 120. The wafer holder 124 is adapted to rotate and press the substrate W against the turntable 122. The wafer holder 124 is connected to a drive shaft 128. The drive shaft 128 is supported by a wafer holder head (not shown) through a pneumatic cylinder so as to be vertically movable.
In this polishing apparatus, the substrate W is held on an elastic mat 130 provided on a lower side of the wafer holder 124 and is pressed against the polishing cloth 120 on the turntable 122. While the substrate W is pressed against the polishing cloth 120, the turntable 122 and the wafer holder 124 are rotated, to thereby effect relative movement between the polishing cloth 120 and the substrate W. During this movement, the abrasive liquid Q is supplied from the abrasive liquid supply nozzle 126 onto the polishing cloth 120. As the polishing liquid Q, for example, use is made of a suspension obtained by suspending fine abrasive particles in an alkali solution. Thus, polishing of the substrate W is conducted by utilizing the effect of chemical polishing using alkali and the effect of mechanical polishing using abrasive particles.
In the above-mentioned polishing apparatus, polishing is conducted by rotating the polishing table 122 about an axis thereof, so that polishing cannot be conducted at the center of rotation where no displacement occurs between the polishing cloth 120 and the substrate W. Therefore, in order to conduct polishing at a position spaced apart from the center of rotation, the size of the polishing table 122 is determined so as to have a diameter which is at least twice the diameter of the substrate. Thus, the polishing table 122 is caused to have a large area, with the result that the polishing apparatus also becomes large and requires costly equipment. This becomes a serious problem with a tendency towards the size of the substrate increasing.
As a countermeasure, it is considered to employ, instead of or in combination with the above-mentioned polishing apparatus, a polishing apparatus in which a polishing table is subjected to a circular orbital motion. In this apparatus, any point on a polishing surface of the polishing table is subjected to the same motion. Therefore, the polishing table is required to have a diameter which is only at least a total of the diameter of the substrate and a value twice the radius of the orbit. That is, the size of the polishing table can be substantially equal to the size of the substrate.
However, in the above-mentioned polishing apparatus, the polishing cloth readily deforms during polishing due to elasticity thereof and enters a space between the projecting portions of the substrate W, so that polishing is conducted with respect to not only the projecting portions, but also the recessed portions therebetween. This leads to undulation of a polished surface of the substrate or difficulty in grinding of the projecting portions of the substrate. As a countermeasure, for example, it has been proposed to conduct polishing by a method using an abrasive plate, which is obtained by binding abrasives such as silica particles with the use of a binder and which is adhered to the polishing table. In this method, polishing is conducted by slidably moving the substrate W held by the wafer holder 124 while pressing the substrate W against the abrasive plate. In this arrangement, during sliding movement of the substrate relative to the abrasive plate, the binder is broken down or melted, to thereby release the abrasive particles. Polishing is conducted by the action of these released particles.
In the above polishing method, the abrasive plate is harder than the polishing cloth, so that the substrate can be polished without undulation occurring. Further, polishing is conducted by using only free particles from the abrasive platte, without using a slurry type abrasive liquid containing a large amount of abrasive particles. Therefore, the amount of abrasive particles used can be reduced, leading to a reduction in the cost of operation and ease of maintenance.
For conducting the above-mentioned circular orbital motion of the polishing table, it is considered to displace the center (center of gravity) of the polishing table from the center axis of the drive shaft and connect the polishing table to an upper end of the drive shaft at the center of the polishing table. In this case, in accordance with the circular orbital motion of the polishing table, a centrifugal force is generated in proportion to the distance between the center axis of the drive shaft and the center of the polishing table, and acts on the polishing table. This causes vibration of the drive shaft. In order to prevent such a vibration of the drive shaft, a counterweight having a center of gravity at a position spaced apart from the center axis of the drive shaft is attached to a predetermined position on the drive shaft, to thereby cancel the centrifugal force acting on the drive shaft.
When the position in an axial (heightwise) direction of the center of gravity of the polishing table and the position in an axial (heightwise) direction of the counterweight are different, a rotational moment is generated and acts on the drive shaft. To cancel the rotational moment, an additional counterweight is provided at different axial position on the drive shaft. However, this leads to an increase in length of the drive shaft and thus in size of the polishing apparatus.
In view of the above, the present invention has been made. It is an object of the present invention to provide a polishing apparatus which can be made compact without impairing the circular orbital motion of the polishing table.
According to one aspect of the present invention, there is provided a polishing apparatus comprising a polishing table having a first axis, the polishing table being adapted to be subjected to a circular orbital motion in which the first axis of the polishing table is rotated about an orbit center axis while the orientation of the polishing table is kept substantially constant, and a counterweight provided on the polishing table for cancellation of a centrifugal force generated by the circular orbital motion of the polishing table. In this apparatus, vibrations imparted to the drive shaft due to the centrifugal force will be able to be suppressed. Further, in this apparatus, there is no need to attach the counterweight directly to the drive shaft and, therefore, the counterweight can be provided at a position (or height) axially close to the polishing table, to thereby reduce the length of the drive shaft and hence, the size of the apparatus.
In accordance with another aspect of the present invention, there is provided a polishing apparatus comprising a polishing table having a first axis, the polishing table being adapted to be subjected to a circular orbital motion in which the first axis of the polishing table is rotated about an orbit center axis while the orientation of the polishing table is kept substantially constant, a cylindrical support member having a center axis and provided around the polishing table, and a stationary bearing provided around the cylindrical member so as to allow the cylindrical support member to rotate about the center axis in response to the circular orbital motion of the polishing table. In this apparatus, due to the stationary bearing constructed as stated above, it will become possible to support the polishing table in a stable condition.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.